I'm getting an unresolved external symbol error in my C++ program. I have provided an MCVE below.
The constructor for the Foo class requires a single int parameter. I want Foo to be a singleton object.
In the Foo class, I have created two accessor functions for retrieving a pointer to the singleton object. The first passes an int parameter to allow construction of the object. The second does not require the parameter, in order to allow the object to be accessed by code that has no knowledge of the parameter.
The error occurs at link time when attempting to create an instance of Foo within main().
Is there a better way to do this, most importantly in order to avoid the error, and (less importantly) so the Foo interface doesn't impose the unfortunate requirement that the second accessor may not be called before the first?
#include <assert.h>
class Foo {
public:
// Accessor function initializes m_param, and
// returns pointer to singleton.
static Foo* GetInstance (int *param)
{
// Foo singleton object is instantiated on first call
// to this function
static Foo instance (param);
pInstance = &instance;
return pInstance;
}
// Overloaded version of GetInstance() that may be called
// after first calling static Foo* GetInstance(int *param).
static Foo* GetInstance ()
{
// This alerts us if the other GetInstance() was not called first.
assert (pInstance);
return pInstance;
}
private:
// constructor
Foo (int *param)
: m_param (*param)
{}
// private var
int m_param;
// pointer to singleton object
static Foo* pInstance;
};
// initialize static var
Foo* pInstance = static_cast<Foo*>(nullptr);
int main(int argc, char* argv[])
{
int pInt;
Foo* pFoo = Foo::GetInstance (&pInt);
return 0;
}
